With a sharply intelligent script that gives the expanding cast plenty to work with, this action epic reverses the diminishing thrills of The Winter Soldier and Age of Ultron. Even if it sometimes feels a bit contrived, the film jolts the Avengers out of their repetitive rut with a much more cinematic adventure that keeps the audience gripped.

As the Avengers save the world, mounting civilian casualties spur the UN to create a council to oversee them. Iron Man Tony (Downey) embraces this, but Captain America Steve (Evans) worries it will diminish the team's ability to help. Then Steve's old pal Bucky (Stan) is framed for a bombing that draws T'Challa (Boseman) into the fray. Battle lines are drawn, as half of the Avengers become outlaws. Tony drafts in Spider-Man Peter (Holland), while Steve recruits Ant-Man Scott (Rudd). And none of them know they're being manipulated by vengeful lone wolf Zemo (Bruhl).

With its nearly two-and-a-half-hour running time, most of these characters get significant time to deal with their own personal issues. And each actor injects some lively personality into the mix. Evans and Downey are superbly spiky at the centre of the storm, while the inclusion of Holland and Rudd adds an entertaining blast of energy. Bettany and Olsen carry the emotional weight, Boseman's Black Panther is both intriguing and darkly engaging, and Renner and Cheadle get the short shrift.

The screenplay cleverly keeps the plot relatively simple, allowing the complexity to emerge within the characters and relationships. This also lets the Russos direct the film without too much confusing cross-cutting, letting scenes play out properly. Although the well-staged action is impossible to see in Imax 3D. Also, there are a few moments when the premise pushes the characters too far, lining up six versus six for the outrageously destructive battle in a conveniently evacuated airport.

Naggingly, it makes no sense that these friends are trying to kill each other. Indeed, the reaction when one goes down betrays this. And even if a later showdown can be explained as an emotional overreaction, it's still uncharacteristic, as formerly smart characters ignore known facts for visceral brutality. But it's still riveting. And it leaves this heroic horde set up for the next episodes in the series. Audiences will be now be impatient for coming stand-alone movies with Boseman and Holland. Not to mention the climactic two-part Infinity War planned for 2018-2019.